In a bus oriented data distribution architecture a plurality of data processing stations are connected to a common data distribution bus. The distribution system is normally operated in two modes of operation: a data transmission mode and a contention mode. During the data transmission mode any particular station transmits data on that bus by bringing the bus into successively different signal or logic states. Similarly, a data driver at a terminal end of a bus may apply data to the bus to be transmitted to one of the data processing stations. In systems where only one data processing station may transmit data at one time, two or more stations may simultaneously compete for access to the bus. A contention mode of operation is utilized to give a particular station priority should two or more data processing stations attempt to transmit data on the bus at the same time.
In a contention mode of operation, two or more individual data processing stations simultaneously apply priority codes to the data distribution bus in order for one of them to gain access thereto: It is essential that the bus be maintained at some predetermined and controlled reference logic state during this mode of operation. This predetermined and controlled reference logic state must establish the reference state for the entire length of the bus and simultaneously permit individual data processing stations connected thereto to override the reference state of the bus in order to place its priority code onto the bus where it may be compared with the priority codes of other data processing stations also competing for access to the bus at the same time.